<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a{
            margin: 20px;
            width: 100px;
            height: 100px;
            background: rgba(0, 0, 0, 0.44);
        }
        .a div{
            margin: 10px;
            width: 30px;
            height: 30px;
            background: #9e73ff;
            border: red;
        }
    </style>
</head>
<body>
<div class="a">
    <div>
    </div>
</div>

</body>
</html>